Of all the entries on this list, the ending of United 93 is the only one where I'm rather self-conciously taking the 'kickass' adjective off (it's an amazing scene, but 'kickass' sort of feels inappropriate). I haven't actually watched the movie since Linds and I saw it in theaters. I probably won't watch it again.
I did just watch the ending again, though, and it's as blood freezing and haunting as ever.